Abstract: A data processing system stores data in a data file by first identifying the data file as a large file type known to require an amount of storage space substantially greater than a system allocation unit size. As data is to be written to the data file beginning at a location identified by a file pointer, the system determines whether it is necessary to allocate storage space of the data processing system to store the data to be written, and if so then commands are issued to the file system including (i) one or more first commands to set an end of file marker for the data file to a new value greater than a current value of the end of file marker by at least an application allocation unit size which is at least twice the system allocation unit size, and (ii) one or more second commands to mark a region of the data file extending between the current and new values of the end of file marker as holding valid data. Subsequently the data is written to the data file at the location identified by the file pointer.